1. Field of the Invention
This invention relates to a method and apparatus for controlling injection molding. More particularly, the invention relates to a method and apparatus for controlling injection molding which uses a mold or molding machine having a temperature adjusting section for adjusting the temperature of a molten resin, wherein control is performed in such a manner that mold temperature, molten resin temperature and molded-article ejection temperature will be held at a predetermined constant temperature or will attain the aforesaid constant temperature in as short a time as possible.
2. Description of the Related Art
In injection molding, maintaining the quality (weight, dimensions, amount of warpage, etc.) of the molded articles is important in order to achieve stable production. However, where there is a fluctuation in the properties of the material (resin) used, a fluctuation in the machinery (the molding machine) or a change in the environment (voltage and current), there is a variation of the state in which the resin is injected into the mold and, hence, a fluctuation in the quality of the molded articles.
Even if the aforementioned fluctuations (the fluctuations in properties, machinery and environment) do not occur, a fluctuation in the quality of the molded articles still occurs in the following cases:
(i) when the molding cycle is changed;
(ii) when there is a decline in cooling efficiency owing to contamination of the mold cooling channels, etc.; and
(iii) when the molding operation is started up (i.e., when the molding machine is started up).
Item (i) above is important when the output (amount of production) of molded articles is changed. For example, if, in a case where output is to be curtailed, the molding machine is shut down after production of the necessary amount of molded articles has been completed, restarting the molding machine requires a great amount of labor. In addition, material is wasted at start-up. In a case where output is curtailed, therefore, it is advisable to lengthen the molding cycle and keep the molding machine in a continuously operating state without stopping it.
The factors which influence the quality of the molded articles will now be described taking the item (i) above, in which the molding cycle is changed, as a typical example.
FIG. 1 illustrates the temperature distribution in a mold at a certain time, wherein a molded article is being cooled during a molding cycle.
Formed within the mold 1 is a cavity 2 filled with a molten resin that has been injected into the cavity. A cooling channel 4 passes in the vicinity of the cavity 2 and carries coolant that flows through it. Temperature is highest at the center of the molten resin within the cavity 2 and declines as the water channel 4 is approached through mold 1. In the description that follows, let the temperature at a prescribed location M near the cavity 2 be represented by mold temperature T.sub.m.
FIG. 2 illustrates the manner in which the mold temperature T.sub.m changes with time in a molding cycle. The molding cycle begins with the injection of the molten resin into the mold. The injection process is followed by a dwell process, which in turn is followed by a cooling process. Though the cooling of the resin within the cavity of the mold begins at the moment of injection, it will be assumed here for convenience sake that the cooling process extends from the moment the dwell process ends to the moment the molded article is extracted from the mold. The mold temperature T.sub.m rises sharply from the moment injection begins, peaks after the dwell process and then gradually declines in the cooling process.
FIG. 3 shows an example of actual measurement of a change in the mold temperature T.sub.m in a case where the molding cycle is changed from 16 seconds to 12 seconds. Since the molding cycle becomes shorter, the mold temperature T.sub.m rises overall. When the molding cycle is lengthened, the mold temperature T.sub.m declines overall. A cooling channel passes through the interior of the mold and coolant flows through the cooling channel, as mentioned above. If the molding cycle is lengthened, the mold is cooled over a longer period of time and the overall mold temperature declines.
The above-described change in mold temperature is the first factor that can be mentioned as having an effect upon the quality of the molded articles in a case where the molding cycle is changed. When a change in mold temperature occurs, this is accompanied also by a change in the state in which the molten resin is injected into the cavity of the mold, thus causing a variation in the quality of the molded articles. For example, the mold temperature declines if the molding cycle is lengthened, as mentioned above. As a result, the molten resin does not fill the interior of the mold as easily and there is a decrease in the weight and dimensions of the molded articles.
A second factor is the temperature of the molten resin injected into the cavity of the mold. For example, when the molding cycle is lengthened in a mold of hot-runner type, the resin resides in the hot runners for a longer period of time in comparison with residence time prior to the cycle change, and therefore the resin is heated for a longer time. Accordingly, the temperature of the molten resin varies nearer to the temperature to which the heaters of the runners have been set, with the resin being injected into the mold cavity at near the set temperature. As a result, there is a variation in the state in which the resin is injected into the cavity (a variation of the state in which the cavity is filled with the resin). This brings about a fluctuation in the quality of the molded articles.
A third factor that can be mentioned is the molded-article ejection temperature. If there is a change in the mold temperature or in the temperature of the molten resin, naturally there will be a change also in the molded-article ejection temperature. When this occurs, the amount of shrinkage of a molded article after molding deviates from the standard, with the result that there is a fluctuation in quality in terms of dimensions and warpage.
A variation in mold temperature and in molded-article ejection temperature is a cause also of a fluctuation in molded-article quality in the case of items (ii) and (iii) mentioned above. Furthermore, the change in the temperature of the molten resin is a cause also of a fluctuation in molded-article quality in the case of item (iii) mentioned above.